The ability to reach toward, grasp, and manipulate objects is nearly unique among primates. The loss of this ability, through stroke or other neurological disease is devastating. Despite increasing knowledge of the number of brain areas participating in the guidance of limb movement, several hypotheses remain regarding the role even of the "primary" motor cortex, a structure that has been recognized for over 100 years, and sends neurons directly to the spinal cord to control movement. The proposed experiments seek to explore the nature of the signals within the primary motor cortex, and the manner in which they relate to muscle activation and the control of arm and hand movements. When a person decides to grasp an object, the motor command is expressed perceptually as a desired movement of the end-point of the limb to a particular point in space. Much of the problem faced by the brain arises from the need to convert this signal into a set of muscle activation signals. Data suggest that the primary motor cortex sends a desired hand movement signal to the spinal cord, and only there is it converted to muscle signals. Other data indicate that the signals sent to the spinal cord have already been converted to muscle activation signals within the motor cortex, or at some earlier stage of processing. Signals will be recorded from the motor cortex and limb muscles of monkeys during limb movement. Hand movement, posture of the limb, and joint torques will also be determined. Varied hand use, altered limb posture, and added inertial loads will be used to decrease the covariation among these signals, and correlation and regression methods will be used to determine their relation to the neuronal signals. In later experiments, signals from several neurons will be used to predict the time course of putative output signals. Motor cortical neurons projecting to the spinal cord will be distinguished from those projecting elsewhere. A neuron with discharge that is consistently related to the activity of a particular set of muscles across the varied types of behavior would be evidence for motor commands in muscle coordinates. On the other hand, discharge that consistently encodes either end-point movement or joint coordinates across tasks, would support the idea that motor cortical command signals are expressed in one of these other coordinate systems. We will attempt to relate functionally distinct groups of neurons to information about their cortical location or output projections.